The advent of miniaturization in the field of electronics has led to the development of programmable processors, such as microprocessors and digital signal processors, which are now commonplace within a wide variety of electronic systems. Each processor typically has an associated a Read-Only Memory (ROM) for storing instructions. The instructions, when executed by the processor, cause the processor to perform one or more functions. Advances in the design and fabrication of such processors now allow them to be fabricated with an internal ROM, thereby avoiding the need for an external ROM for program storage as was required in the past.
Usually, the internal ROM contained in today's processors is mask-programmed. In other words, the ROM within the processor is programmed by appropriately arranging the transmissive and non-transmissive portions of one or more masks which are employed to photolithographically define each layer of the processor during fabrication. Since the programming of the internal ROM is an integral part of the fabrication of the processor, modifying or altering the program requires that a new processor be fabricated. Often, the lead time required to obtain a new processor with a differently mask-programmed ROM is relatively long.
When developing an electronic system incorporating a processor, there is often a need during development to reprogram the ROM associated with the processor. Indeed, the program contained within the ROM may undergo a large number of alterations or modifications before a final "production" version of the electronic system is created. For this reason, an external ROM is often employed for program storage in place of the internal ROM during development to facilitate such frequent reprogramming.
However, in some instances, it may not be possible to employ an external ROM because of space requirements. With many products, the amount of space available for mounting components is limited, with little extra room for an external ROM that will be used only during system development. Without the ability to employ an external ROM, a designer may be forced to utilize the internal processor ROM, thus incurring long lead times when the program in the ROM must be changed.
This, there is a need for an electronic module which occupies the same area (i.e., "footprint") as a processor, but contains both the programmable processor and a reprogrammable ROM, the latter being separable from the processor to facilitate off-line reprogramming thereof.